1. Field of the Invention
The present invention relates to an apparatus and method for controlling a switch of a flyback converter for a solar generating system, and more particularly, to an apparatus and method for controlling a switch of a flyback converter for a solar generating system capable of simplifying a circuit while solving disadvantages of a discontinuous conduction mode and a boundary conduction mode, by controlling a magnetizing current flowing into a transformer of a flyback converter in the discontinuous conduction mode or the boundary conduction mode, based on a predetermined conduction ratio limit value.
2. Description of the Related Art
Generally, a solar generating system implies a system that uses a solar cell to convert light energy into electrical energy. A general configuration of a solar generating system including a flyback converter is shown in FIG. 1. Referring to FIG. 1, the solar generating system is configured to include a power converter 20, such as a flyback converter, connected to a solar cell module 10 and a power grid 30 connected to an output stage of the power converter 20.
As shown in FIGS. 2A to 2B, in the solar cell module 10 used in the solar generating system, the amount of generated maximum power and the conditions (a maximum power point 11) generating maximum power are fluctuated according to insulation, peripheral temperature, or the like. In other words, FIG. 2A shows characteristics that the maximum power point 11 of the solar cell module 10 is increased with the increase in insulation and the maximum power point 11 of the solar cell module 10 is reduced with the increase in the peripheral temperature. Therefore, the power converter 20, such as a flyback converter for a solar cell alternating current generating system, should be designed to output a maximum output selected under various input conditions.
Meanwhile, the foregoing flyback converter 200 is controlled by a separate controller, wherein the controller controls a magnetizing current of a transformer by using a switch control method of a boundary conduction mode BCM and a discontinuous conduction mode DCM.
The discontinuous conduction mode is a switch control method discontinuously controlling the magnetizing current of the transformer. According to the discontinuous conduction mode, since the internally used carrier has a fixed frequency, the structure of the controller is simple. However, there is a problem in that the maximum power is limited by magnetizing inductance of a transformer. In order to overcome these disadvantages, a method to reduce the magnetizing inductance of the transformer is used, which has a problem in reducing conversion efficiency.
On the other hand, the boundary conduction mode BCM is a switch control method controlling the magnetizing current without generating a section in which the magnetizing current flowing into the transformer is continued to 0. Since the boundary conduction mode BCM controls the magnetizing current without generating a section where the magnetizing current is continued to 0, a switching frequency controlling a main switch is changed and a switching frequency is maximized in a specific portion. There is an advantage in that the change in the switching frequency does not limit the maximum output power according to the magnetizing inductance value of the transformer. However, there are problems in that a separate current detection circuit is required and a circuit for generating the switching signals is complicated. In addition, since the switching frequency is maximized in the specific portion, there is a problem in that switching loss and burning within the main switch may be caused.